Whisk together the flour and salt in a large bowl. Using the large end of a box grater, shred the frozen butter right into the bowl with the flour and stir to coat as you go. Stir the water and cider vinegar together. Add the water mixture to the flour mixture 1-2 tablespoons at a time, mixing with a fork until the dough starts to clump together.
Cut the dough in half, form it into two disks, wrap both in separate pieces of plastic, and chill in the refrigerator for at least 2 hours or up to overnight.
After the dough has thoroughly chilled, gently roll out one of the disks (leave the other one in the fridge) to a 12-inch circle on a floured surface. Then fold the piecrust or roll it around your rolling pin to move it to a 9-inch pie dish. Gently tuck it into the pie dish, trim the edge about an inch over the side, tuck it in along the edge, and flute it.
Preheat oven to 425 degrees.
Mix together the blueberries, sugar, cornstarch, cinnamon, and lemon juice in a large bowl. Spoon it into the prepared crust. Cut the butter into small pieces and sprinkle over the top of the berries.
Roll out the other dough disk on a floured surface. Using a small cookie cutter like a heart or star, cut shapes from the rolled-out dough and arrange them on the pie. I did it in five rows that went in opposite directions for every other row.
Using a fork, beat the egg yolk and water together. Brush the egg wash lightly over the crust. Place the pie on a baking sheet. Bake for 20 minutes. Then decrease to 375 degrees and bake for 25-30 minutes or until the pie filling is bubbly and the crust is golden brown. If the top is browning too fast, simply cover it with a piece of aluminum foil loosely on the top.
Allow the pie to cool for several hours on a wire rack before slicing to allow the filling to thicken up.